[Bug-XBoard] XBoard external pixmaps

[h.g. muller]

Some people that are using XBoard with external pixmaps complain
that they get error messages for the new piece types, like

Error -1 loading XPM file "/home/roy/xboard/merida/cocoa/fll72.xpm"

Now this error should be non-fatal, but I can imagine that people get
annoyed because they suddenly get multiple error messages where
before they got none. This does violate the guideline of reproducing
old behavior as closely as possible.

Shall I still make a patch in xboard.c to suppress this error message
when the piece type is "non-essential", and replace it by loading
the corresponding king pixmap? Missing pixmaps of un-orthodox
pieces would then not give error messages, but people would
discover soon enough that something is amiss when they start
to play variants containing these pieces. This behavior would
furthermore be similar to that when using built-ins for certain board
sizes, which is described in manual, and as a warning in the
New Variant dialog.

I think this is an easy patch with not much risk to break anything.
So I could still do it before we tag as bet2 (or 4.4.0), and it could
save us a lot of angry complaints.